The Boil in the Brewing Process

One of the most neglected procedures in homebewing is the boil. After all, it's only boiling the wort, right? Well, yes and no. Believe it or not, boiling the wort accomplishes quite a few things. These include:

It extracts the hop α-acids, isomerizes them, then dissolves them.

It stops all enzyme activity.

It also kills bacteria, wild yeast and fungi in your wort.

It gets rid of undesirable hop oils that may be too harsh as well as sulfur compounds, keytones and esters that may also be too harsh for your beer.

It causes excess proteins to combine with polyphenols and coagulate which forms the hot break.

Helps with the formation of melanoidins and it also caramelizes enough of the wort sugars to get some color and toffee-like flavors if the boil is vigorous and long enough.

And lastly, it concentrates the excess water in the wort so you can hit your pre-fermentation original gravity and allows you to extract as much of the sugars in the mash as you safely can (before they turn too harsh from extraction of polphenols from the husks).

Hop α-acids generally take at least 60 minutes to extract an appreciable amount. Boiling for 90 or 120 minutes will increase the amount somewhat, but then you run the risk of adding too much color and possibly too much kettle caramelization for the style you are brewing. Sixty minutes is all that is really needed for extract brewing but for homebrewers making all grain beer, you may need to boil at least 90 minutes to get rid of the precursors to DMS. Be sure you leave the lid off your boil kettle so the DMS precursors can escape, otherwise you may find unwanted flavors in your beer.

It also takes a good rolling boil for at least an hour so the hop compounds can combine with the polypeptides in the mash to form colloids which aid in head formation and retention.

Removal of the hot break material is very important for beer clarity. The proteins which cause cloudiness and chill haze must be removed with the hot break material in the bottom of the boil kettle. If left in the beer, the proteins will attract any bacteria you may have inadvertently introduced during bottling or transfer. Certain hop compounds which help preserve the beer will be lost as well if the boil does not last at least an hour as it takes that long to extract them from the hops in the wort.

There are times when your mash pH going into the kettle is on the low side, say 5.2 or 5.1. Since the act of boiling wort will reduce the pH somewhat as the calcium and phosphates in your grain combine and precipitate, you run the risk of leaving proteins in your wort which will later cause clarity problems and possibly some fermentation issues as well. Along with these problems you also run into losing some hop utilization from the lower pH of the wort. Taking a pH reading with a meter is important for the advanced brewer for the reasons listed above. When you find your pH on the low side, adjust it with chalk or calcium carbonate prior to the boil.

There are some styles, Scottish ales and bocks for instance, that benefit from some melanoidin formation and the caramelization that occurs in a vigorous boil. There are a few tricks brewers use to increase the rich malty flavor and even the caramel flavors of some styles. One is to take a small portion of the first runnings and boil the heck out of them. A vigorous boil will caramelize the sugars and concentrate the flavors. This is then added back to the boil with the rest of the mash to yield a darker color and nice melanoidin-rich flavor which may imitate a long decoction mash if done correctly. One caveat to remember, sometimes kettle caramelization can seem like diacetyl to some judges so be aware of the problem and be forewarned.

All in all, the boil is a very important part of your brew day and you can use it to control many aspects of the flavor and aroma of your beer.

References: Information for this article was adapted in part from the article Hop Utilization & Cool Brewings written by Ashton Lewis for BYO magazine in the April 1999 issue, and Section C. Wort Production written by David Houseman and Scott Bickham of the Interim Study Guide for the BJCP Exam.